The present disclosure relates to x-ray angiography and, in particular, the disclosure relates to a system and method for resolving artifacts in time-resolved, three-dimensional (3D) angiographic images, referred to as four-dimensional (4D) angiographic x-ray data, such as may be caused by overlapping anatomical features.
Since the introduction of angiography beginning with the direct carotid artery punctures of Moniz in 1927, there have been ongoing attempts to develop angiographic techniques that provide diagnostic images of the vasculature, while simultaneously reducing the invasiveness associated with the procedure. In the late 1970's, a technique known as digital subtraction angiography (DSA) was developed based on real-time digital processing equipment. Due to steady advancements in both hardware and software, DSA can now provide depictions of the vasculature in both 2D and volumetric 3D formats. Three-dimensional digital subtraction angiography (3D-DSA) has become an important component in the diagnosis and management of people with a large variety of central nervous system vascular diseases as well as other vascular diseases throughout the body.
In recent years competition for traditional DSA has emerged in the form of computed tomography angiography (CTA) and magnetic resonance angiography (MRA). CTA is a less invasive technique but has lower spatial resolution. It is not time-resolved unless the imaging volume is severely limited. The images are not isotropic and secondary reconstruction yields degraded spatial resolution. CTA is also somewhat limited as a standalone diagnostic modality by artifacts caused by bone at the skull base and as well as the contamination of arterial images with opacified venous structures. Further, CTA provides no functionality for guiding or monitoring minimally-invasive endovascular interventions.
Significant advances have been made in both the spatial and the temporal resolution qualities of MRA. Currently, gadolinium-enhanced time-resolved MRA (TRICKS) is widely viewed as a dominant clinical standard for time-resolved MRA. TRICKS enables voxel sizes of about 10 mm3 and a temporal resolution of approximately 10 seconds. Advancements such as HYBRID highly constrained projection reconstruction (HYPR) MRA techniques, which violate the Nyquist theorem by factors approaching 1000, can provide images with sub-millimeter isotropic resolution at frame times just under 1 second. Nonetheless, the spatial and temporal resolution of MRA are not adequate for all imaging situations and its costs are considerable. Furthermore, the spatial and temporal resolution is substantially below other methods, such as DSA.
The recently-introduced, four-dimensional (4D) DSA techniques can use rotational DSA C-arm imaging systems controlled with respect to a particular injection timing so that there is time dependence in the acquired reconstructed 4D volumes. As described in U.S. Pat. No. 8,643,642, which is incorporated herein by reference, a 3D DSA volume can be used as a constraining volume to generate a new 3D volume that contains the temporal information of each projection. As in 3D DSA, a mask rotation without contrast is followed by a second rotation during which contrast is injected. The process creates a series of time resolved 3D angiographic volumes that can be updated, for example, every 1/30 of a second.
Thus, the above-described systems and methods have improved over time and, thereby, provided clinicians with an improving ability to visualize the anatomy of the vessels being studied. While 4D DSA techniques present a great advancement in the resources available to clinicians, 4D DSA images can include artifacts caused when anatomical features overlap in a 2D time frame x-ray projection. These artifacts can occur because the intensity information in overlapping anatomical features, at times, cannot be uniquely divided amongst the individual features. That is, existing 4D DSA methods reconstruct the 3D volume from the collection of 2D projections, and a time-resolved sequence of volumes is generated by back-projecting the information from the 2D frames onto the 3D volume. Overlapping anatomical features in a 2D projection, in some cases, cannot be perfectly distinguished and, thus, the intensity information cannot be attributed with confidence to individual features in the 3D volume. Thus, these overlapping features can cause uncertainty and inaccuracies in the 4D DSA reconstruction, which manifest as artifacts.
Therefore, it would be desirable to have systems and methods that are able create images, such as 4D DSA images, without artifacts caused by overlapping anatomical features.